Well, a great plot idea might be delving deeper into the relationship between Batman and his rogues gallery in the aftermath. For example, how the villains react to Batman's continued presence or absence. Some might see it as an opportunity to take over Gotham, while others might be secretly glad he's still around to keep a certain order. Also, exploring the technological fallout. With all the advanced tech used during Arkham Knight, there could be some unforeseen consequences like tech falling into the wrong hands or new, unexpected uses for it.
One idea could be exploring how the Knightfall event affects Batman's relationships with his allies in the Arkham Knight universe. Maybe some of them start to question his methods more deeply.
One interesting plot idea could be a fanfiction where the Joker somehow comes back to life in Arkham Knight. Maybe he was only in a coma and wakes up with a new, even more deranged plan to take over Gotham. He could start by manipulating the inmates in Arkham again, creating chaos from within the asylum.
In this fanfiction, it could be interesting if Catwoman discovers a secret about the Arkham Knight that Batman has overlooked. She could use this knowledge to her advantage, either to make a deal with the Arkham Knight or to help Batman. For example, she might find out that the Arkham Knight has a weakness related to a certain location in Gotham that was important to his past. Catwoman could then lead Batman there to take down the Arkham Knight.
A great plot idea for this fanfiction could be that Harley Quinn is haunted by the memories of her time with the Joker during the Arkham Knight scenario. Batman, noticing her distress, tries to help her. As they interact more, Harley starts to see Batman in a new light and decides to assist him in his fight against the Arkham Knight. Along the way, they face many challenges, like dealing with Harley's old gang who don't trust her new alliance with Batman, and also the Arkham Knight's devious traps. This would make for a complex and engaging fanfiction story.
Another plot could involve Weiss using her family's resources to create technology similar to what's in Arkham Knight's world to combat a new type of Grimm. And the Arkham Knight himself might be interested in this tech and try to steal it, leading to an interesting conflict and alliance between Weiss and the other RWBY characters against him.
One idea could be exploring the backstories of the lesser - known characters in the 'Batman Assault on Arkham' universe more deeply. For example, you could focus on Killer Croc's life before Arkham and what led him to become the way he is. Another plot could involve a team - up between unexpected characters from the movie, like Harley Quinn and Deadshot, against a new threat in Arkham that the Bat - family is unaware of.
One idea could be a story where Batman and Catwoman team up to take on a new, more powerful villain in Arkham. Maybe this villain has the ability to control the minds of the inmates, creating chaos. They'd have to use their unique skills, Batman's detective work and combat abilities, and Catwoman's agility and thieving expertise to stop the threat.
One idea could be exploring a new villain in Neo - Gotham. Maybe a tech - based criminal who can hack into the Bat - suit's systems, creating a whole new set of challenges for Terry McGinnis. Another could be a story where Terry has to team up with an old - school Batman villain who has somehow resurfaced in the future, like the Riddler, but with a new high - tech twist on his riddles.
Well, you could write a fanfiction where the Joker somehow returns in Arkham Beyond, but he has a new, more terrifying form. Batman must figure out how this is possible and stop him before he causes even more mayhem. Or perhaps a story centered around Harley Quinn trying to redeem herself in the Arkham Beyond setting. She could team up with Batman to take down a common enemy, which would create an interesting dynamic between the two characters.
Another plot could involve a crossover with a fantasy world. Imagine Batman being transported to a world like Middle - earth. He'd have to use his gadgets and combat skills in a world full of magic and mythical creatures. He might team up with characters like Aragorn or Legolas to fight against an evil force that has some similarities to the criminals in Arkham Asylum, perhaps a dark sorcerer who is as mad and unpredictable as the Joker.